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Establishing the origin of electrophoretic mobility of sarcoplasmic reticulum (SR) vesicles is the primary goal of this work. It was found that the electrophoretic mobility originates from ionizable amino acids of cytoplasmic domains of the Ca2+–ATPase, the calcium pump of SR. The mobility was measured at pH 4.0, 4.7, 5.0, 6.0, 7.5, and 9.0 in the region of ionic strength from 0.05 to 0.2 M. Mobility measurements were supplemented by studies of SR vesicles by photoelectron microscopy. The median diameter of SR vesicles was 260 nm. Ca2+–ATPases were not resolved. The mobility data were standardized by interpolation to a reference ionic strength of 0.1 M. The mobility of the SR vesicles is determined by the charge of the Ca2+–ATPase. It is due to the ionizable amino acids selected from the amino acid sequence of SERCA1a Ca2+–ATPase. The pH dependence of charge residing in various domains of Ca2+–ATPase was computed using pKa values in free water. The charge correlated with measured mobility. It was shown that a linear relationship exists between the mobility of the SR vesicles, μ, and the total computed charge, Q, on three cytoplasmic domains of Ca2+–ATPase: A, P, and N. It is given by μ = α + βQ where the fitted values β = (0.043 ± 0.002) × 10−8 m2 V−1 s−1 e−1 and α = (0.16 ± 0.02) × 10−8 m2 V−1 s−1. Since β and α values do not change from pH 4 to pH 9, one concludes that the hydrodynamic friction of the cytoplasmic domains of SR is independent of their charge.